1. Field of the Invention
This invention relates to light emitting diodes (LEDs) and in particular to high efficiency and high brightness LEDs for various lighting applications, and methods of fabricating the same.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Gallium nitride (GaN)-based wide band gap semiconductor light emitting diodes (LEDs) have been available for almost 15 years. The progress of LED development has brought about great changes in LED technology, with the realization of full-color LED displays, LED traffic signals, white LEDs and so on.
Highly efficient white LEDs have gained much interest as possible replacements for fluorescent lamps. For example, the luminous efficacy of white LEDs (130-150 lumens/watt [1]) already surpasses that of ordinary fluorescent lamps (75 lumens/watt.) Nevertheless, current commercially available wurtzite nitride-based LEDs are characterized by the presence of polarization-related electric fields inside multi-quantum wells (MQWs), for their [0001] c-polar growth orientation. The discontinuities in both spontaneous and piezoelectric polarization at the heterointerfaces result in internal electric fields in quantum wells which cause carrier separation (quantum confined Stark effect (QCSE)) and reduce the radiative recombination rate within quantum wells [2-5]. To maintain a decent radiative recombination rate, c-polar light emitting devices typically have thin (<3 nm) quantum wells [6-7].
To decrease these polarization-related effects, growing III-nitride devices on the non-polar planes (viz, the (1-100) m-plane or the (11-20) a-plane) has been demonstrated [8-9]. Another approach to reduce, and possibly eliminate, those effects is to grow III-nitride devices on crystal planes that are inclined with respect to the c-direction, i.e., semi-polar planes. These planes have reduced polarization discontinuity in heterostructures, compared with the c-plane III-nitride materials; and for semi-polar planes oriented ˜45° from the c-plane, there is no polarization discontinuity in InGaN/GaN heterostructures [5]. With reduced polarization-related electric fields inside the quantum well region, the electron and hole wavefunctions inside a semi-polar-oriented InGaN quantum well are expected to have more overlap (and thus lead to a higher radiative efficiency) than in a c-polar oriented counterpart, for a given quantum well thickness. In other words, without worrying about the detrimental effect on the radiative recombination rate, one can employ thick quantum well designs in semi-polar LEDs. Devices grown on different semi-polar planes (including (10-1-1), (10-1-3), (11-22) planes etc.) have been demonstrated and they exhibited greatly reduced polarization-related electric fields [10-12]. The output powers of those heteroepitaxially-grown devices, however, suffer from the presence of stacking faults and threading dislocations. Recently, with the advent of high quality freestanding GaN substrates, high performance non-polar and semi-polar LEDs with peak emission wavelengths ranging from 407 nm to 513 nm on non-polar m-plane, semi-polar (10-1-1), and (11-22) freestanding GaN substrates have been reported [13-17]. Nonetheless, the output powers of those devices are still lower than that of typical the-state-of-the-art c-polar devices, which can be partly attributed to the un-optimized LED epitaxial layer structure.
This invention presents novel semi-polar LED epitaxial layer structures that are expected to address this problem.